1. Field of the Invention
The present invention relates to wire bonding methods, semiconductor devices, capillaries for wire bonding and methods of forming ball bumps, and, more particularly, to a wire bonding method, a semiconductor device, a capillary for wire bonding and a method of forming ball bumps which provide an efficient and reliable wire bonding.
Generally, semiconductor elements provided in a semiconductor device are electrically connected, using a wire, to a lead serving as an external connection terminal. Such a wire is disposed by a wire bonding unit to extend between a pad formed on a semiconductor element and an inner lead of the lead.
The number of wires disposed is equal to the number of pads formed on the semiconductor element. With increasingly higher degree of integration, an increasing number of pads are provided in semiconductor elements. As a result, the number of wires disposed between the pad and the inner lead becomes increasingly large.
Improvement in productivity and reduction of size are major requirements in manufacturing a semiconductor device. Reduction of time needed to wire-bond individual wires lends itself to improvement in productivity, while lowering of a loop height of wires lends itself to reduction of size.
2. Description of the Prior Art
FIGS. 1 and 2 show wires disposed according to the conventional wire bonding. FIGS. 3A-3C and FIGS. 4A-4C show a detailed wire bonding method.
FIG. 1 shows a wire 1 disposed using the most generally practiced wire bonding method. Referring to FIG. 1, the wire 1 is disposed between a pad 2a formed in a semiconductor element 2 and an inner lead 3a of a lead frame 3.
In order to dispose the wire 1 between the pad 2a and the inner lead 3a, the wire 1 is first bonded to the pad 2a (first bonding). In this case, the ball bonding as shown in FIGS. 3A-3C is employed in the first bonding.
As shown in FIGS. 3A-3C, a metal wire forming the wire 1 is led through a capillary 7 provided in a wire bonding unit. Referring to FIG. 3A, a ball-shaped part 4 is formed at the head of the wire 1 by spark discharge. Then, as shown in FIG. 3B, the ball-shaped part 4 is pressed against the pad 2a using the capillary 7 and bonded therewith using the ultrasonic welding process.
Then, the wire 1 which extends to the inner lead 3a is formed by a guiding process effected by guiding by drawing the capillary 7 in a direction indicated in FIG. 3C. The wire 1 is bonded to the inner lead 3a (second bonding). In this case, the stitch bonding process shown in FIGS. 4A-4C is employed in the second bonding.
Referring to FIG. 4A, the end of the capillary 7 is guided by drawing to a bonding position of the inner lead 3a. Subsequently, as shown in FIG. 4B, the end of the capillary 7 is pressed against the inner lead 3a so that the wire 1 is bonded to the inner lead 3a in the ultrasonic welding process. The portion of the wire 1 which is pressed by the capillary 7 is deformed as a result of the capillary 7 being pressed against the inner lead 3a. 
Thereafter, the capillary is guided upward. While the capillary is guided upward, a clamper 8 provided in the wire bonding unit holds the wire 1 fixed. Therefore, the wire 1 is cut at the portion thereof that is deformed and has lost the mechanical strength.
As has been described, according to the most generally practiced conventional wire bonding method, the wire 1 is provided between the pad 2a of the semiconductor element 2 and the inner lead 3a of the lead frame 3 as shown in FIG. 1 through the first bonding that uses the ball bonding process and the second bonding that uses the stitch bonding process (hereinafter, such a method of providing the wire 1 is referred to as forward bonding).
A description will now be given, with reference to FIGS. 5A-6B, of the characteristic of the ball bonding process and the stitch bonding process.
FIG. 5A is a perspective view showing how the wire 1 is bonded to the pad 2a using the ball bonding process, and FIG. 5B is a top view of the bonded area. As has been described already, the ball bonding process is performed such that the ball-shaped part 4 is formed in the wire 1 using spark discharge and then bonded to the pad 2a. As a result of such a process, the bonded area at the bottom of the ball-shaped part 4 has a circular shape in a top view and resides within the pad 2a. 
FIG. 6A is a perspective view showing how the wire 1 is bonded to the inner lead 3a using the stitch bonding process, and FIG. 6B is a top view of the bonded area. As shown in the figures, since the wire 1 is pressed by the capillary 7 according to the stitch bonding, the wire 1 is flattened into a bonded portion 9 having a relatively wide area. For comparison, FIGS. 6A and 6B include broken lines indicating the pad 2a. Characteristically, the stitch bonding process requires a wider bonding area as compared to the ball bonding process.
In the forward bonding described above, the wire 1 is guided upward after being bonded to the pad 2a and then bonded to the inner lead 3a. Hence, the height of the loop formed by the wire 1 is relatively high with respect to the upper major surface of the semiconductor element 2.
In the construction shown in FIG. 1, the wire 1 is higher in level than the upper major surface of the semiconductor element 2 by a height H. Accordingly, the forward bonding has a problem in that it is difficult to make the semiconductor device thin.
FIG. 2 shows a wire 5 provided according to a bonding method proposed to eliminate the problem resulting from the forward bonding. In FIG. 2, those components that are the same as the components of FIG. 1 are designated by the same reference numerals.
As shown in FIG. 2, the wire 5 is provided such that the ball-shaped part 4 is formed at the end of the wire 5 jutting out of a capillary using spark discharge. The ball-shaped part 4 is then pressed against the inner lead 3a of the lead frame 3 so as to be welded thereon according to the ball bonding process.
The capillary is then guided upward to a level slightly higher than the upper major surface of the semiconductor element 2 and then guided horizontally so that the wire 5 is guided to a position above the pad 2a. The capillary is then pressed against the pad 2a so that the wire 5 is bonded to the pad 2a using the stitch bonding process. More specifically, a ball bump 6 formed of a gold or the like is provided in advance on the pad 2a, and the wire 5 is bonded to the ball bump 6.
It will be noted that the bonding order of the bonding method shown in FIG. 2 is different from that of the bonding method shown in FIG. 1 in that wire is bonded to the inner lead 3a in the first bonding and then bonded to the pad 2a (more specifically, to the ball bump 6 on the pad 2a) in the second bonding. The method for providing the wire 5 will be referred to as a backward bonding.
According to the backward bonding, wire 5 is first bonded to the inner lead 3a which is lower in level than the upper major surface of the semiconductor element 2. The wire 5 is then guided upward to reach the same height as the upper major surface of the semiconductor element 2. Thereafter, the capillary is guided horizontally so that the wire 5 can be bonded to the pad 2a (more specifically, the ball bump 6).
As shown in FIG. 2, the wire 5 provided between the pad 2a and the inner lead 3a is bent to form an approximately right angle (an inverted L shape) so that the loop formed by the wire 5 is lower than that of the construction shown in FIG. 1. Accordingly, it is possible to make the semiconductor device thin.
While the backward bonding described above ensures that the semiconductor device produced has a relatively small height because the loop-formed by the wire 5 is relatively low, it has a problem in that the ball bump 6 must be formed on the pad 2a using a gold or the like.
The ball bump 6 serves as a buffer for preventing the pressure of the capillary from being exerted on the semiconductor element 2 directly when the wire 5 is bonded to the pad 2a using the stitch bonding process. The ball bump 6 also has a function of improving the resistance of the wire bonding.
If the wire 5 is directly bonded to the pad 2a using the stitch bonding process without using the ball bump 6, the semiconductor element 2 may be damaged due to the pressure exerted by the capillary. Insufficient bonding of the wire 5 to the pad 2a may cause the wire 5 to be removed from the pad 2a. For this reason, it is necessary to provide the ball bump 6 on the pad 2a in the conventional backward bonding.
The conventional backward bonding has a problem in that a process of providing the ball bump 6 is needed in addition to the process of providing the wire 5, adding to the number of processes in the wire bonding, and in that the efficiency in producing the semiconductor device is unsatisfactory.
The ball bonding process employed in forming the ball bump 6 is such that a gold ball is formed at the end of a gold wire and then bonded to the pad 2a, after which the gold wire is cut. In this process of forming the gold ball, a pressure is exerted on the semiconductor element 2.
As has been described, the bonding of the wire 5 to the ball bump 6 involves a pressure exerted on the semiconductor element 2. Therefore, the semiconductor element 2 undergoes the pressure on two occasions during the wire bonding process, that is, when the ball bump 6 is formed and when the wire 5 is bonded. Thus, even if the ball bump 6 is provided, a possibility remains that the semiconductor element 2 is damaged during the whole wire bonding process.
As has been described with reference to FIGS. 5A-6B, the stitch bonding process requires a wider bonding area as compared to the ball bonding process. Therefore, it is impossible to bond the wire 5 on the upper major surface of the semiconductor element 2 with a high density in the backward bonding. The stitch bonding process is not opted for if the semiconductor element 2 has pads 2a disposed thereon with a high density.
For this reason, only the forward bonding shown in FIG. 1 can be applied to the high-density semiconductor element 2. Accordingly, the semiconductor element 2 has a relatively large size for the aforementioned reason.
As has been described, the backward bonding is useful for the semiconductor element 2 that does not have pads 2a disposed thereon with a high density. However, an upper surface 6a (the surface to which the wire 5 is bonded) of the ball bump 6 is rugged because it is the cut surface of the gold wire. Therefore, the bonding strength that exists between the wire 5 and the ball bump 6 is not satisfactory so that the reliability of the wire bonding is not satisfactory.
Further, as shown in FIG. 8, if an especially large bump 6b is produced on an upper surface of the ball bump 6, the wire 5 is bonded at a position displaced from the center of the ball bump 6. An edge part 5a characterized by a low bonding strength and produced as a result of the wire 5 deformed flat by the capillary 7 resides on the end of the upper major surface of the ball bump 6. The wire 5 may be cut at a portion adjacent to the edge of the upper major surface of the ball bump 6.
It is thus an object of the present invention to provide a semiconductor device having a wire loop with an L shape or curved shape. In this present invention, the semiconductor device includes a semiconductor element; a lead; a wire connecting the semiconductor element and the lead, wherein the lead is directly bonded to the wire using a first ball-shaped part formed in the wire. An electrode formed in the semiconductor element is directly bonded to the wire using a second ball-shaped part formed in the wire. The wire forms a loop having a curved shape and including a vertical portion that extends vertically from a position where the wire is bonded to the lead, and a horizontal portion that extends horizontally from a position where the semiconductor element is bonded to the wire.
Accordingly, it is an object of the present invention to provide a wire bonding method, a semiconductor device, a capillary for wire bonding and a method of forming ball bumps in which the aforementioned problems are eliminated.
Another and more specific object of the present invention is to provide a wire bonding method, a semiconductor device, a capillary for wire bonding and a method of forming ball bumps which provide an efficient bonding process, reduction in damage to connected bodies, and reduction in the height of semiconductor elements.
In order to achieve the aforementioned objects, the present invention provides a wire bonding method comprising:
a first bonding process for forming a first ball-shaped part in a wire and bonding the first ball-shaped part to a first connected member;
a ball-shaped part forming process for guiding the wire away from the first connected member so as to form a predetermined loop and forming a second ball-shaped part in a predetermined position in the wire; and
a second bonding process for bonding the second ball-shaped part to a second connected member.
According to the wire bonding method of the present invention, a damage is prevented from occurring in a semiconductor element when the wire is bonded to the second connected member because the second ball-shaped part functions in the same manner as a ball bump of the backward bonding process. Since the second ball-shaped part is formed in the wire, no additional process separate from the sequence of the wire bonding process is necessary in order to form the ball bump. Accordingly, the wire bonding process can be carried out efficiently. The process for bonding the second ball-shaped part to said second connected member is a so-called ball bonding process and requires a relatively small area. Hence, a wire bonding process can be securely carried out in a semiconductor element characterized by a high density with which electrodes are provided.
Preferably, in the ball-shaped part forming process, the second ball-shaped part is formed while the wire remains uncut. According to this aspect of the present invention, it is possible to position the second ball-shaped part on the pad in a reliable manner. Accordingly, the second ball-shaped part can be properly bonded to the pad.
Preferably, at least the second ball-shaped part is formed by spark discharge. According to this aspect of the present invention, no modification to the existing wire bonding unit is necessary because the spark discharge process is normally practiced to form the first ball-shaped part.
In another preferred embodiment, the wire is guided vertically upward after the first ball-shaped part is bonded to the first connected member; and the wire is then guided horizontally, forming a right angle, whereupon the second ball-shaped part is formed and bonded to the second connected member. According to this aspect of the present invention, the wire provided between the first connected member and the second connected member is low. Therefore, the height of the semiconductor device produced can be kept low.
In order to attain the aforementioned objects, the present invention provides a wire bonding method comprising the steps of:
providing a first wire according to the wire bonding method as claimed in claim 1;
forming a third ball-shaped part in a second wire and bonding the third ball-shaped part to the second connected member; and
guiding the second wire away from the second connected member so as to form a loop above the first wire, and stitch-bonding the second wire to the first connected member.
According to this wire bonding method, the second wire can be easily provided above the first wire because the loop formed,by the first wire is low. Accordingly, it is possible to provide wires with a high density and to prevent the first and second wires from interfering each other.
In another preferred embodiment, the first connected member is embodied by a lead frame, and the second connected member is embodied by a semiconductor element. Accordingly, the semiconductor device can be thin and the production efficiency thereof is improved as compared to the conventional method.
In still another preferred embodiment, the first connected member and the second connected member are embodied by a semiconductor. Therefore, the multi-chip module produced according to the present invention can be thin and the production efficiency thereof is improved as compared to the conventional method.
In yet another preferred embodiment, the wire is embodied by a thin gold wire, and a ball bonding process is employed to bond at least the second ball-shaped part to the second connected member. According to this aspect of the present invention, a damage is prevented from being exerted on the second connected member.
In order to attain the aforementioned objects, the present invention also provides a semiconductor device comprising:
a semiconductor element;
a lead; and
a wire connecting the semiconductor element and the lead;
wherein
the lead is directly bonded to the wire using a first ball-shaped part formed in the wire,
an electrode formed in the semiconductor element is directly bonded to the wire using a second ball-shaped part formed in the wire, and
the wire forms a loop having an L shape and including a vertical portion that extends vertically from a position where the wire is bonded to the lead, and a horizontal portion that extends horizontally from a position where the semiconductor element is bonded to the wire.
According to the semiconductor device of the present invention, a damage exerted on the semiconductor element is reduced because the electrode formed on the semiconductor element is directly bonded to the wire using the second ball-shaped part formed in the wire. By providing the wire so as to form a L-shaped loop, it is possible to reduce the height of the semiconductor device.
In order to attain the aforementioned objects, the present invention also provides a wire bonding capillary for use in a bonding process for bonding a wire to a connected member, wherein an end of a main body of the capillary includes a projection. According to the wire bonding capillary of the present invention, the end portion projecting from the main body of the capillary acts to bond the wire to the ball bump and the portion surrounding the projecting end portion acts to press the ball bump.
Preferably, a bonding part for bonding the wire to the connected member is formed at the end of the main body of the capillary, and a pressurizing part for pressing a ball-shaped part formed by the bonding part in the connected member is formed adjacent to the bonding part. Accordingly, the process for bonding the wire to the connected member and the process for welding the ball-shaped part formed in the connected member can be carried out using the same capillary. Hence, the forming of the ball bump can be carried out efficiently.
In order to attain the aforementioned objects, the present invention also provides a ball bump forming method using the wire bonding capillary as claimed in claim 10, the ball bump forming method comprising:
a bonding process for forming the ball-shaped part in the wire and bonding the ball-shaped part to the connected member using an end portion of the wire bonding capillary;
a ball bump forming process for forming a ball bump on the connected member by cutting the wire by drawing the wire bonding capillary away from a position where the wire is bonded to the connected member; and
a shaping process for turning an upper major surface of the ball bump flat by causing a portion of the wire bonding capillary which portion surrounds the end portion to press the ball bump.
Further, the present invention provides a ball bump forming method using the wire bonding capillary as claimed in claim 11, the ball bump forming method comprising:
a bonding process for forming the ball-shaped part in the wire and bonding the ball-shaped part to the connected member using the bonding part provided in the wire bonding capillary;
a ball bump forming process for forming a ball bump on the connected member by cutting the wire by drawing the wire bonding capillary away from a position where the wire is bonded to the connected member; and
a shaping process for turning an upper major surface of the ball bump flat by causing the pressurizing part provided in the wire bonding capillary to press the ball bump.
According to the ball bump forming method of the present invention, the upper major surface of the ball bump formed in the connected member can be turned flat. Thus, the quality of bonding that exists between the ball bump and the wire can be improved, resulting in a highly resistant bonding. The bonding process, the ball bump forming process and the shaping process can be effected in a successive manner using the same capillary. Hence, the ball bump forming process according to the present invention is highly efficient.